1. Field of the Invention
The present invention relates to a wireless communication device, a communication method, and a storage medium, with which communication is performed by switching mutually different directivity response patterns formed by an adaptive array antenna.
2. Description of the Related Art
Millimeter wave wireless communication is a technique for realizing high-speed, large-capacity wireless communication. A characteristic of radio waves in the millimeter wave band is a high degree of straightness, so a problem is that communication tends to be disconnected if a person or other obstacle moves into the propagation path. A conventional attempt to deal with this problem was to transmit data using a plurality of different propagation paths between a wireless transmitter and a wireless receiver.
Japanese Patent Laid-Open No. 2000-165959 proposes a method for simultaneously receiving a direct wave and one or more reflected waves, each at the proper strength. With this method, even if one propagation path should be cut off, radio waves will arrive from other propagation paths, so the necessary received signal strength can be maintained. However, since radio waves are received simultaneously from a plurality of propagation paths, a new problem arises in that depending on the environment in which the wireless transmitter and wireless receiver are installed, there may be no room for the effect of frequency-selective fading.
As shown in FIG. 1, another method is to repeatedly transmit a signal produced from the same source data while switching the directivity response pattern in a wireless communication system including a wireless transmitter 110 and a wireless receiver 120 equipped with an adaptive array antenna. For example, let us term directivity response patterns having a main lobe in the direct wave direction a first transmission directivity response pattern 111 and a first reception directivity response pattern 131. Let us also term directivity response patterns having a main lobe in the direction in which radio waves arrive in a single reflection off of a wall or other such reflecting object 150 a second transmission directivity response pattern 112 and a second reception directivity response pattern 132. The redundancy of communication can be improved by repeatedly transmitting a signal using these two sets of directivity response pattern.
With a wireless communication system such as this, a test signal is transmitted ahead of data transmission, and the received signal strength thereof is observed. This makes it possible to select the first transmission directivity response pattern 111 and first reception directivity response pattern 131, and the second transmission directivity response pattern 112 and second reception directivity response pattern 132. However, even when using a directivity response pattern in which a direction free of any reflecting object 150 is used as the main lobe, if the side lobe level is high in the direct wave direction, a direct wave can still be transmitted and received at a sufficiently high power. Accordingly, as shown in FIG. 2, there will be cases in which a directivity response pattern having a relatively large side lobe in the direct wave direction is selected as the second transmission directivity response pattern 112 and the second reception directivity response pattern 132. This is particularly likely to happen when the distance from the reflecting object 150 is relatively far, the directivity response pattern has a main lobe in the reflecting object 150 direction, and reflected waves are received only at a relatively low power.
In this case, communication using the second transmission directivity response pattern 112 and the second reception directivity response pattern 132 is performed by a side lobe in the direct wave direction. Accordingly, a problem arises in that if an obstacle moves into the direct wave direction, communication with the first transmission directivity response pattern 111 and the first reception directivity response pattern 131, and communication with the second transmission directivity response pattern 112 and the second reception directivity response pattern 132 are simultaneously cut off.
Because of limitations on the time allowed for training of directivity response patterns, and mounting limitations such as the resolution of a phase shifter provided to an adaptive array antenna, the directions which can become a main lobe are discrete. Accordingly, the reception power may be higher with a directivity response pattern in which the maxima of the side lobes coincide in the direct wave direction than with a directivity response pattern in which the maxima of the side lobes are closest to the direct wave direction but offset slightly. In this case, unlike with the above-mentioned problem, communication with the first transmission directivity response pattern 111 and first reception directivity response pattern 131 is performed with a side lobe of the direct wave direction. Furthermore, when a directivity response pattern in which the maximum of the main lobe is closest to the direct wave direction is selected as the second transmission directivity response pattern 112 and the second reception directivity response pattern 132, communication ends up being performed using a direct wave for both sets of directivity response pattern. Accordingly, if a malfunction should occur in one communication path due to interruption of the direct wave or another such situation, communication with the two sets of directivity response pattern may be simultaneously cut off.
The present invention was conceived in light of the above problem, and provides a technique for utilizing a plurality of communication paths with different directivity response patterns.